InAs self-assembled quantum dots (SAQDs) were grown on GaAs(1 0 0) substrates using a molecular beam epitaxy (MBE) technique. The InAs QDs were multi-stacked with 6,10 and 15 layers in which 2 monolayer (ML) thick InAs QD layers and 20 ML thick GaAs spacers were alternately positioned. The nanostructural features of the QD multi-stacked layers were characterized by scanning transmission electron microscopy (STEM). The QDs were well formed vertically up to 6-7 layers in each multilayered structure. Additionally, stacking faults, "volcano-like" defects and defects of an asymmetrical down triangular shape (ADTS) were observed in the 10- and 15-layer InAs QD/GaAs samples. The generations of these defects are discussed mainly in terms of the interdiffusion of the III-group atoms, Ga and In. The subsequent strain relaxation and gradient of surface chemical potential are suggested to be the cause of these defects.